The Virginia ponies are treated to twice-yearly veterinary inspections, which prepare them for life among the general equine population if they are sold at auction.

This poor-quality (and often seasonally-scarce) food source—combined with uncontrolled inbreeding—created a propensity for conformation faults in the Chincoteague; new bloodlines began to be introduced in the late 19th and early 20th centuries.

In general, the breed tends to have a straight or slightly concave facial profile with a broad forehead and refined throatlatch and neck.

[3] In the late 19th century, one author praised their "good manners and gentle disposition" while reporting the story of one pony who was ridden a distance of around 1,000 miles (1,600 km) in 34 days by a man with equipment, a load that weighed around 160 pounds (73 kg)—the pony weighed approximately 500 pounds (230 kg).

[7] In 1896, Leonard D. Sale wrote in The Horse Review of Chicago that an Assateague resident imported and released a "Canadian-bred pony stallion" in an effort to improve the breed.

[7] The influence of the Arabian horse breed on Chincoteague ponies continues to be seen today, with many Chincoteague ponies having the refined, wedge-shaped heads, a broad forehead, large eyes, large nostrils, and small muzzles – or the "concave, dished profile" – seen with many Arabian crosses.

[8] Legend states that Chincoteague ponies descend from Spanish horses shipwrecked off the Virginia coast on their way to Peru in the 16th century.

They argue that horses were too valuable in the 17th century to have been left to run wild on the island, and claim that there are two sunken Spanish galleons off the Virginia coast in support of their theory.

[12][13] In the early 1900s, they were described as having been on the islands since well before the American Revolution, and were described at that time as "very diminutive, but many of them are of perfect symmetry and extraordinary powers of action and endurance".

Misshapen legs, narrow chests, poor bone and a lack of substance plagued the breed, with many stunted animals not growing above 12 hands (48 inches, 122 cm).

This was partially due to the limited and poor-quality feed found on the islands, although this harsh habitat also allowed only the hardiest and most adaptable ponies to survive.

[16] As a result of the jetties disrupting sand movement in the area, the island has drifted westward, and the two landmasses are now over 1 kilometer (0.62 mi) apart.

[15] In 1835, the first written description of "pony penning" (roundup) appeared, though the practice of rounding up livestock on the island existed for many years before that.

[5]: 287–290 In 1909, the last Wednesday and Thursday of July were designated as the annual days for pony penning, still taking place on both Assateague and Chincoteague Islands.

[5]: 287–290 As many as 50,000 visitors gather on the last Wednesday in July to watch mounted riders bring the Virginia herd from Assateague and swim them across the channel to Chincoteague Island.

[citation needed] The swim takes five to ten minutes, with both the rider and the observers on hand to assist horses, especially foals, who may have a hard time with the crossing.

Though descended from the same original stock, the Maryland feral ponies are called "Assateague horses" and are maintained by the National Park Service.

[27]: 48–50  Ponies on Assateague have a diet that consists mainly of cordgrass, a coarse grass that grows in salt marshes, which makes up around 80 percent of their food.

This diet is supplemented by other vegetation such as rose hips, bayberry, greenbriar, American beach grass, seaweed and poison ivy.

Since the late 1970s, scientists have used the herd to conduct studies on feral horse behavior, social structure, ecology, remote contraceptive delivery and pregnancy testing, and the effects of human intervention on other wild animal populations.

There are few other wildlife populations of any species worldwide that have been studied in as much detail over as long a period as the Maryland herd of Chincoteague ponies.

To manage population numbers, long-term, non-hormonal contraceptives have been employed, proving 95 percent effective over a seven-year field trial.

[5]: 287–290  The contraceptive, which began to be used at a management level in 1995 although it was used in smaller amounts as early as 1989, has also proven effective at improving the health and increasing the life expectancy of older mares through the removal of pregnancy and lactation-related stress.

No horse has ever been injured during the dart-administered treatments, although there is a 0.2 percent rate of abscess at the injection site, which normally heals within two weeks.

[29] Other than the contraceptive and treatment in emergencies, ponies from the Maryland herd are treated much like other wildlife, with no extra attention paid to them by Park Service employees.

It is thought likely that the Maryland herd carries equine infectious anemia (EIA); they are effectively quarantined, however, by allowing no riding or camping with privately owned horses along the mainland shore during the insect season which stretches from mid-May to October.

[3] Due to their treatment as wild animals, ponies from the Maryland herd can be aggressive, and there have been reports of them tearing down tents and biting, kicking and knocking down visitors.

[5]: 287–290  Since 1943, the FWS has been working on the island to protect and increase the wildfowl population, and their efforts have sometimes endangered the Chincoteague herd.

[27]: 48–50  Since 1990, the ponies from the Virginia herd have been rounded up biannually for veterinary treatment, including deworming and vaccinations for diseases such as rabies, tetanus and Eastern and Western encephalitis, although they make the swim to Chincoteague only once per year.

Such intervention is needed because many of the ponies will be brought into the general horse population through the auction and purchase by private buyers.